There are various apparatus and methods known for generating gaseous bubbles in liquids. Additionally, there are numerous processes which require gaseous bubble generation, forced air induction, solution aeration or liquid gas dispersion. One such process comprises the concentration and benefication of mineral ore material by froth flotation. A froth flotation process includes forming an aqueous slurry or pulp containing a mixture of mineral and gangue particles. One or more flotation agents is added to the pulp to promote flotation of either the mineral or the gangue. The pulp is aerated by introducing a plurality of minute air bubbles which become attached to either the mineral particles or the gangue particles and cause these particles to rise to the surface of the pulp. U.S. Pat. Nos. 3,371,779 and 4,287,054 to Hollingsworth et al and 4,247,391 to Lloyd disclose several froth flotation methods and apparatus.
Flotation efficiency is influenced by the size of bubbles supplied to the flotation cell or column. For example, as bubble size decreases, the surface area per unit volume increases whereby more surface area is available for particle attachment. Additionally, bubbles form a wake as they rise and draw unattached particles with them. Thus, the larger the bubble, the faster the rise rate and the greater the amount of material drawn up with the bubble. However, bubble-particle hydrodynamics also suggest that many fine particles may never collide with excessively large bubbles because the fine particles, being low in mass, cannot deviate from the stream line and are swept away as the larger bubbles pass. Accordingly, it is important to control the size of bubbles which are supplied to a flotation column.
While various means are known for generating bubbles for a flotation column or cell, many of the known methods and apparatus suffer various disadvantages. For example, mechanical air induction means, such as pumps, impellers and orifices are known. When using a pump, however, if the air is aspirated in a negative pressure situation, the volume percent of air in the liquid is limited by the pump's capacity to compress the gas in solution and maintain a positive pressure at its outlet. Therefore, if the gas content of the pump feed water exceeds the pump's capacity, the pump will cavitate causing it to lose pressure at its outlet and cease pumping. This limit is generally reached well below the 5% gas in water minimum content needed for column flotation. Additionally, mechanical air induction means often require excessive amounts of chemical frother in order to stabilize the fine bubbles. However, large amounts of frother are chemically detrimental to the flotation separation process, particularly when mineral slimes are included. When using mechanical air induction means, a large recycling load may also be required in order to increase the number of fine bubbles in solution. However, large recycling is energy intensive and inefficient.
High shear impellers are often used for providing bubbles in flotation columns and cells. U.S. Pat. Nos. 1,124,855 to Callow et al and 4,231,974 to Engelbrecht et al discloses impeller means for providing gaseous bubbles. However, the turbulence caused by such impellers negate the benefit of quiescent separation conditions within a flotation column. Additionally, orifices on impellers often plug rapidly due to the presence of solids in the column and, since they are located within the columns, are difficult to repair and replace.
Pressure dissolution means are also known for generating gaseous bubbles in a liquid. However, pressure dissolution means may achieve only a limited volume percent of gas dissolved in water, usually less than 1%, because of the limited solubility of most gases in water. This limit is well below the volume percent of gas necessary for column flotation. The gas solubility may be increased by increasing pressure, which is expensive and requires special high-pressure equipment, or by selecting a more soluble gas than air, such as carbon dioxide, which is an expensive alternative. An additional alternative for increasing gas solubility is decreasing the temperature of the bubble generating system. However, this is also a costly alternative.
Alternatively, electrolytic bubble generation means may be used as disclosed in the Kikindai et al U.S. Pat. No. 3,479,281. However, electrolytic bubble generation is disadvantageous in that it produces flammable hydrogen gas, produces oxygen which may oxidize mineral surfaces, decreases flotation efficiency and often requires the addition of salts to lower solution resistivity. Additionally, electrolytic bubble generation does not produce the large volumes of gas necessary for column flotation and requires large amounts of electrical energy. Finally, only small bubbles in the range of 0.8 to 1.0 mm diameter sizes may be produced using electrolytic bubble generation means and the bubble size may only be controlled by frother addition.
Additional sparger bubble generation systems are disclosed in the Sumiya U.S. Pat. No. 3,032,199 wherein air conduit pipes having small openings for uniformly introducing air into a flotation cell are provided near the bottom of the cell. This type of bubble generator is disadvantageous in that the small openings tend to plug due to scale formation or particulate blocking. Since the bubble generation means is located within the flotation cell, it is difficult to service. Additionally, the bubble generator does not provide for bubble size control except by frother addition. An additional bubble generator is disclosed in the Flynn U.S. Pat. No. 4,512,888.
Thus, there is a need for a bubble generator for supplying gaseous bubbles in a liquid to a flotation cell or column, which bubble generator provides bubble size control without requiring excessive frother additions, is external to the cell or column, and is inexpensive.